Neurones in the dorsolateral periaqueductal grey matter in coronal slices of rat midbrain: electrophysiological and morphological characteristics

Abstract

 Intracellular recordings using the whole-cell patch-clamp configuration were made from 87 neurones in the dorsolateral ”aversive” region of the periaqueductal grey matter (PAG) in coronal slices of rat midbrain. Camera lucida reconstructions made of 28 of the cells, which had been filled with biocytin, revealed round, triangular or oval cell bodies 11–40 µm in diameter. Between two and seven primary dendrites were present, which branched further, often becoming varicose. The dendritic tree was always contained within the dorsal half of the PAG. Biocytin-filled axons could be followed for 177–2315 µm from the soma. The axons typically travelled to the perimeter of the dorsal half of the ipsilateral PAG, before either turning ventrolaterally to run parallel to the tecto-bulbopsinal fibres or continuing their trajectory into the deep collicular layers or the mesencephalic reticular formation. Electrophysiologically, two functional categories of cells could be distinguished: type-A cells (30%) showed inward rectification at membrane voltages in excess of –77 mV and had short action potentials (1.6±0.07 ms), which were followed by a biphasic afterpolarisation characterised by an initial fast and a later slow phase. Only 18.5% of the type-A cells were spontaneously active (<4 Hz). The second category of cell (type B, 70% of the recorded population) did not show rectification and had longer-duration action potentials (2.1±0.07 ms), which showed a smooth decay of the afterhyperpolarisation phase. Approximately one third (37%) of the type-B cells fired spontaneously (<4 Hz). The gross morphology of the two types of cells was similar. However, in type-A cells (n=6), the axons could be seen to originate from the cell soma, whereas, in the majority of the type-B population (10/13), the axon arose from a primary dendrite. The results show that coronal slices of midbrain contain a viable population of ”output” or ”projection” neurones, which are accessible to electrophysiological and pharmacological investigation. In conscious animals, efferent output from this part of the PAG is concerned with mediating the autonomic and somatomotor changes which are characteristic components of aversive emotional behaviour. The output neurones should therefore reflect the net level of excitability in the PAG in relation to its functional activity.